The present invention relates generally to microwave ovens including supplementary electrical resistance heating capability and, more particularly, to such an oven which is adapted for operation from a power source insufficient to supply both the microwave and the electrical resistance heating capabilities simultaneously at their respective full rated power levels.
Ovens employing microwave energy to rapidly cook food have come into widespread use in recent years. While microwave cooking generally has the advantage of being faster than conventional cooking, it has long been recognized that conventional cooking is superior in certain respects. In particular, for some types of food, microwave cooking is considered unsatisfactory by many people for the reason that there is usually only a slight surface browning effect, especially where a relatively short cooking time is employed. Additionally, foods such as steaks, chops, or the like, of relatively small thickness, are often more satisfactorily cooked when rested on a plate heated sufficiently to cause searing. Similarly, foods which are cooked in relatively shallow metal utensils, such as frozen dinners, are more advantageously cooked when the metal utensile itself is heated.
To realize the benefits of both methods, a number of combination microwave and conventional cooking ovens have been proposed and commercially produced. These ovens, as their name implies, combine in a single cavity the capability of microwave cooking and conventional cooking by electrical resistance heating. The microwave cooking capability is provided by a microwave energy source such as a magnetron which produces cooking microwaves when energized from a suitable high voltage DC source. Means for providing conventional cooking capability may take any one of a number of forms including sheathed electrical resistance heating elements, commonly called broil and bake elements, at the top and bottom of the cooking cavity respectively; heaters applied to utensil-supporting plates; and forced convection designs which include a fan for circulating air past a heating element and then across the food.
Several of these designs have proven to be quite satisfactory in operation and commercially successful. They are typically full-size combination conventional and microwave ovens operated from a 240 volt power source with a current-supplying capability which, for practical purposes, is unlimited. Therefore, simple switching schemes may be employed to alternately energize either the microwave cooking capability, the conventional cooking capability, or both capabilities simultaneously. Many thousands of watts of power are available from the power source, and this is sufficient to heat a domestic sized cooking oven in any manner desired.
More recently, so-called countertop microwave ovens have been introduced. These ovens typically have a somewhat smaller cooking cavity compared to a full-size conventional oven and are designed for operation from a 115 volt, 15 amp household branch circuit. To meet UL requirements, an appliance designed for operation from such a power source is limited to a maximum requirement of 13.5 amperes. This corresponds to approximately 1550 watts. As explained next, this limited power souce capability results in some particular problems.
A typical microwave energy generating system intended for a countertop microwave oven requires a major portion of this available power. Such a typical system comprises a magnetron which produces between 500 and 600 watts of output power at a frequency of 2450 MHz, and a suitable power supply for the magnetron. A typical microwave energy generating system has an energy conversion efficiency in the order of 50%. In addition to the microwave energy generating system, a practical microwave oven includes a number of low power load devices such as lamps, motors, and control circuitry. Altogether, one particular commercially-produced countertop microwave oven model draws approximately 11.2 RMS amps from a 115 volt line for microwave cooking alone. This corresponds to approximately 1300 watts.
In addition, supplementary electrical resistance heating units, for effective operation, should be operated at approximately 1200 to 1400 watts. This is particularly so for infrared food surface browning. For effective and reasonably rapid browning, the watts density over the area covered by a supplementary electrical resistance browning element should be approximately 20 watts per square inch. With 1200 watts of available browning power, approximately 60 square inches could be covered by radiation from such a browning element. Even 60 square inches is a relatively small area, and any decrease in available browner power would reduce the covered area even further. As a result, where a browner unit is provided, substantially all of the limited available power should be supplied to the browning unit.
Therefore, for an oven designed for operation from a 115 volt, 15 amp household branch circuit, as a practical matter the limited power available precludes the simultaneous energization of the microwave energy generating system and the supplementary electrical resistance heating units at their respective full rated power levels.
In answer to this practical limitation on available power, countertop microwave ovens intended for operation from a power source insufficient to supply both the microwave and electrical resistance browning capabilities simultaneously at their respective full rated power levels have resorted to a two-step cooking procedure whereby cooking by microwave energy is accomplished first, with the electrical resistance browning element de-energized. Next the microwave source is de-energized and the electrical resistance browning element is energized for the remainder of the cooking cycle.
As an alternative to a separate electrically energized heating element for browning or the like, a number of special utensils have been proposed and commercially produced to effect browning when used in a microwave oven. These utensils comprise an element, for example a thin resistive film applied to an undersurface of the utensil, which element has the capability of absorbing some of the microwave energy available in the cooking cavity and converting the same to heat. The utensil itself becomes elevated to a sufficiently high temperature for browning or searing. In a similar vein, devices have been proposed which alter the electromagnetic energy within the cooking cavity so as to produce near field dielectric heating for improved surface browning. It will be appreciated that, while such utensils are beneficial with certain foods, the microwave energy they absorb is then unavailable for direct heating of the food. Additionally, they are not as efficient as direct electrical resistance heating because the less-than-100% energy conversion efficiency of the microwave energy generating system must be taken into account.
While not directly related to browning, an important feature included in many microwave ovens is a variable microwave power level control. Variable power level control provides flexibility in cooking various types of food, including thawing frozen foods at a reduced power level. One particular power level control scheme which is employed in microwave ovens is duty cycle power level control whereby the microwave energy source is pulsed from full OFF to full ON repetitively, with the duty cycle under control of the user of the oven. In this way, the time averaged rate of heating can be effectively controlled. The repetition period may vary from in the order of one second for fully electronic duty cycle power level controllers, to in the order of thirty seconds for electromechanical cam operated duty cycle power level controllers.